1. Field of the Invention
The present invention relates to object scanning and specifically to the electrical interface and design of a fingerprint sensor.
2. Prior Art
Fingerprint scanners are one form of biometric verification used to access data and devices. Devices that have potential use for biometric verification include laptop computers, personal digital assistants, cellular or mobile telephone handsets, and any other device requiring access restriction or security. Mobile telephone handsets commonly incorporate a camera for capturing still and moving images but are not suitable for fingerprint scanning due to the optics, image area, and lack of proper excitation to measure ridge and valleys of fingerprints.
Fingerprint sensing technology is well known. One method uses an alternating current excitation signal that propagates an electric field through the finger while a capacitive sensor detects the ridges and valleys in the living layer of skin beneath the skin surface. A fingerprint scanner is typically a two-dimensional sensor array with a pixel width count much larger than the pixel height count. Pixel dimensions of 128 by 8 or 12 and 192 by 16 are common, and other pixel sizes and aspect ratios can be used. The finger is manually swiped across the sensor.
Consecutive images are captured as the fingerprint passes across the sensor and are combined to form a full image of the fingerprint.
Standard fingerprint scanners interface to a host device using a specialized interface intended for the scanner or a general purpose interface such as universal serial bus (USB) See Lei et al., U.S. patent application publication US 2006/0069826 published Mar. 30, 2006 for an example of a fingerprint sensor that interfaces to an external computing device over USB.
A specialized interface for the sensor precludes compatibility with existing devices that are not designed with a physical layer interface suitable for connection to the sensor. A USB interface in a fingerprint sensor requires significant die area on the device and thereby increases cost of the sensor.
Fingerprint scanners have special mechanical and electrical needs due to the exposure of the active sensor area. The sensor must be exposed for the finger to contact with the sensor. Electrostatic discharge (ESD) protection must be provided on the sensor chip, which uses die area around the active sensor region.
Reduction of the number of interface signals is highly desirable. Each input or output (I/O) pin requires a bonding pad on the integrated circuit, which uses die area and thus increases cost. The pad can be used for connection to a bump or a bond wire. Additionally, mechanical clearance distances are required between the bonding pads and the active sensor array. Reduction in the number of pads reduces clearance requirements and reduces die size.
An interface to standard processor devices is desirable to increase compatibility and avoid redesign of existing processors. A low cost sensor is desirable, which requires minimizing die area and I/O pad count.